1. Field of the Invention
This invention relates to a method for removing hydrogen sulfide from gas at room temperature and normal pressure, which is a wet desulfurization method.
2. Description of the Related Art
The methods of gas desulfurization are generally divided into two categories. One is called dry desulfurization with a high desulfurization rate, while the other is called wet desulfurization with high processing capacity, but low desulfurization rate.
Wet desulfurization is a common desulfurization method used in different fields, with a very similar desulfurization method.
The alkanolamine purification method is the most commonly used desulfurization method in the field of oil refining. Desulfurization with the alkanolamine purification method is a typical absorption-regeneration reaction method. In this method, the weak alkaline solution is used as the absorbent when absorbing the hydrogen sulfide in the absorption tower (the carbon dioxide and other composition containing sulphur are absorbed at the same time). The water solution (rich liquid) is desorbed in the regeneration tower after being heated under normal air pressure, so as to regenerate the solution. The poor liquid after regeneration is cooled and transferred into the absorption tower for recycling, and the acid gas from the top of the absorption tower is transferred to sulphur recycling device after condensation and liquid separation for recycling the sulphur.
The disadvantage of alkanolamine purification method are as following:                (1) The alkanolamine compounds are with high vapour tension, so that it can easily be taken away by the gas purified.        (2) The alkanolamine compound solution is apt to sparkle. It may cause the difficulty of handling, so as to decrease the processing capability of the device, increase the losses of solvent, and decrease the purification rate.        (3) The method is not selective to the acid gas, and also works on carbon dioxide.        (4) The method gains a low desulfurization rate.        
During the course of gas production with coal as the raw composition, the gas produced contains considerable H2S and a small amount of organic sulphur and unsaturated hydrocarbon. Several wet desulfurization methods that are commonly used are described as below:
1. Rectisol Wash Method
The rectisol wash method is one of the physical absorption methods in wet desulfurization methods. The principle of this method is that the compositions used such as CO2, H2S, Organic sulphur, cyanide, and unsaturated hydrocarbon are highly dissolved in methanol under high air pressure and low temperature, and can be desorbed from the methanol when air pressure decreases, so that the compositions can be removed from the gas. The method comprises the following steps: firstly, raw gas at the low section of the absorption tower is washed with methanol solution at the temperature of −70° C.; secondly, the CO2 and the H2S in the raw gas are absorbed and the temperature of the methanol rises; the solution flows out of the absorption tower from the bottom section, and enters the top section of the methanol regeneration tower; at the moment, the pressure drops; then partial CO2 and H2S are desorbed out due to the drop of pressure and overflows from the top of the tower, meanwhile, the methanol solution is cooled down to the temperature of −35° C. In the regeneration tower, with the solution flowing downwards, the air pressure drops to 0.02 MPa, then the CO2 and the H2S desorbed are pumped out by a pump, meanwhile, the methanol solution is cooled down to −70° C.; finally, the CO2 and the H2S are transferred by the pump to the top of lower section of the absorption tower for recycling.
The disadvantage of this method is that the method needs to be conducted at a high air pressure and low temperature, in addition, the process is very complicated, has a very high requirement for the composition quality of device, and may cause a huge loss of methanol solution.
2. The Improved Stretford Method (ADA)
The Stretford method is a chemical absorption method in wet desulfurization method. The dilute sodium carbonate solution is only added with anthraquinone disulphonate as the desulfurizer; wherein, sodium carbonate solution is used as the absorbent, and anthraquinone disulphonate is used as the catalyst. In order to improve the reaction rate, a certain amount of vanadate is added to the desulfurizer, so that it is called the improved ADA. The improved ADA started to be adopted from 1960's and has become one of the main gas desulfurization methods.
The detailed method of the improved ADA comprise the following steps: Firstly, the gas enters the absorption tower from the bottom section of the tower, the desulfurization solution is sprayed into the tower from the top section of the tower, and the H2S in the gas is absorbed by the solution via gas-liquid countercurrent contact; Secondly, the gas desulfurized overflows from the top of the absorption tower. Meanwhile, air is introduced into the bottom of the regeneration tower, so that the solution can be regenerated; then the regenerated solution is transferred into the absorption tower for recycling; after that, the sulphur foam collected in the reaction groove is put in the collection groove; Finally, the sulphur foam is pressed into the sulphur foam groove by mean of compressed air.
The disadvantages of this method are described as below:                (1) It may cause considerable loss of sodium carbonate, and needs to provide a sustaining supply thereof.        (2) During the recycling of the solution, more and more sodium hyposulfite and sodium thiocyanate without the value in use will be collected, so as to drop the effect of desulfurization.        
3. The Sulfolane Method
In the sulfolane Method, the blended water solution of sulfolane and alkylalkanolamine is used as the absorbent, and the acid gas such as CO2 and H2S is physically dissolved in the sulfolane. Under the same circumstance, the H2S in the sulfolane solution gains a solubility seven times of that in water. The method is a physical absorption method belonging to wet desulfurization method. As the solution contains 20%-30% ethanolamine, an unstable complex compound is generated after the reaction between ethanolamine and H2S, so that this absorption method is the combination of physical absorption method and chemical method. Under a certain air pressure, the absorbing ability of sulfolane may increase with the rising of the air pressure of the acid gas.
The working pressure in the absorption tower used in the sulfolane method is generally 7.0 MPa, so that it is not commonly used in desulfurization under a high air pressure.
4. The Selexol Method.
In the selexol method, Polyethylene glycol dimethyl ether is used as the absorbent. Selexol is the product name of Polyethylene glycol dimethyl ether. The absorption in this method is conducted under a high air pressure, so that this method now is eliminated.
Above all, the wet desulfurization method in the prior art generally requires a harsh desulfurization condition and high quality devices, and has a complicated procedure; in addition, it causes the problems such as a considerable loss of consumption, and very bad effect of desulfurization. Furthermore, it has been mentioned in a lot of documents that amorphous iron oxide hydroxide can be used as the desulfurizer, but the content of amorphous iron oxide hydroxide in the amorphous iron oxide hydroxide desulfurizer produced via the prior art is very low (approximately 40%). However, the content of other iron oxides, that are not regeneratable, such as ferriferous oxide and ferric oxide and other iron oxide hydroxide is very high, and this causes these amorphous iron oxide desulfurizers not to be regeneratable, so that it may not be adopted in wet recycling desulfurization.